In commonly assigned U.S. Pat. No. 4,584,592 issued on Apr. 22, 1986 in the names of Hsing C. Tuan and Malcolm J. Thompson entitled, "Marking Head For Fluid Jet Assisted Ion Projection Imaging Systems", there is disclosed a marking array for use in conjunction with the marking head of an ion projection printer of the type disclosed in commonly assigned U.S. Pat. No. 4,463,363 issued on July 31, 1984 in the names of Robert W. Gundlach and Richard L. Bergen, entitled, "Fluid Jet Assisted Ion Projection Printing". In that printer, an imaging charge is placed upon a moving receptor sheet, such a paper, by means of a linear array of closely spaced minute air streams. Charged particles, comprising ions of a single polarity (preferably positive), are generated in an ionization chamber of the marking head by a high voltage corona discharge and are then transported to and through the exit region of the marking head, where they are electrically controlled, at each image pixel point, by an electrical potential applied to a modulating electrode. Selective control of the modulating electrodes in the array will enable spots of charge and absence of charge to be recorded on the receptor sheet for subsequent development.
A large area marking head for a page-width line printer would typically measure about 8.5 inches wide. A high resolution marking array capable of printing 200 to 400 spots per inch would, therefore, include about 1700 to 3400 conductive metallic modulation electrodes. The entire array measuring on the order of 8.5 inches by 0.7 inches also would include a multiplexed addressing assembly comprising metallic address lines and data lines and amorphous silicon thin film active switching elements. All of these elements would be fabricated upon a single low cost substrate, such as glass.
During the operation of such an ionographic printer there is an outflow of corrosive agents from the ionization chamber. These agents have a propensity to attack the exposed modulation electrodes very rapidly, thereby lowering the operational lifetime of the marking array. Heretofore, the modulation electrodes have been fabricated of inexpensive electrically conductive materials which are compatible with standard thin film deposition techniques and which may be also used for conductive lines and for contacts with the active devices. Typically, this material has been aluminum. It has been observed that aluminum modulation electrodes oxidize rapidly, resulting first in changed electrical characteristics since the aluminum oxide is insulating and not conductive, and finally in catastrophic electrical and mechanical failure as the electrodes are fully converted to the brittle insulating oxide which flakes off the substrate. An inert material, such as gold, has yielded extremely corrosion resistant electrodes but its cost and non-compatibility with the marking head fabrication process has negated its practical use. In a copending patent application, filed concurrently herewith, entitled "Modulation Electrodes Having Improved Corrosion Resistance" in the names of Nicholas K. Sheridon and Henry Sang Jr. (identified by Attorney Docket No. D/87083), there is disclosed a marking array comprising modulation electrodes fabricated of an aluminum/copper alloy having improved corrosion resistance in the ionographic environment.
Therefore, it is the primary object of this invention to protect the modulation electrodes from the corrosive ionographic effluents by imposing a constant low level bias voltage on them relative to the marking head.